Metal Surface Engineering for Extreme Sustenance of Jumping Droplet Condensation.

Water vapor condensation on metallic surfaces is critical to a broad range of applications, ranging from power generation to the chemical and pharmaceutical industries. Enhancing simultaneously the heat transfer efficiency, scalability, and durability of a condenser surface remains a persistent challenge. Coalescence-induced condensing droplet jumping is a capillarity-driven mechanism of self-ejection of microscopic condensate droplets from a surface. This mechanism is highly desired due to the fact that it continuously frees up the surface for new condensate to form directly on the surface, enhancing heat transfer without requiring the presence of the gravitational field. However, this condensate ejection mechanism typically requires the fabrication of surface nanotextures coated by an ultrathin (<10 nm) conformal hydrophobic coating (hydrophobic self-assembled monolayers such as silanes), which results in poor durability. Here, we present a scalable approach for the fabrication of a hierarchically structured superhydrophobic surface on aluminum substrates, which is able to withstand adverse conditions characterized by condensation of superheated steam shear flow at pressure and temperature up to ≈1.42 bar and ≈111 °C, respectively, and velocities in the range ≈3-9 m/s. The synergetic function of micro- and nanotextures, combined with a chemically grafted, robust ultrathin (≈4.0 nm) poly-1H,1H,2H,2H-perfluorodecyl acrylate (pPFDA) coating, which is 1 order of magnitude thinner than the current state of the art, allows the sustenance of long-term coalescence-induced condensate jumping drop condensation for at least 72 h. This yields unprecedented, up to an order of magnitude higher heat transfer coefficients compared to filmwise condensation under the same conditions and significantly outperforms the current state of the art in terms of both durability and performance establishing a new milestone.

Ultrathin Durable Organic Hydrophobic Coatings Enhancing Dropwise Condensation Heat Transfer.

Organic hydrophobic layers targeting sustained dropwise condensation are highly desirable but suffer from poor chemical and mechanical stability, combined with low thermal conductivity. The requirement of such layers to remain ultrathin to minimize their inherent thermal resistance competes against durability considerations. Here, we investigate the long-term durability and enhanced heat-transfer performance of perfluorodecanethiol (PFDT) coatings compared to alternative organic coatings, namely, perfluorodecyltriethoxysilane (PFDTS) and perfluorodecyl acrylate (PFDA), the latter fabricated with initiated chemical vapor deposition (iCVD), in condensation heat transfer and under the challenging operating conditions of intense flow (up to 9 m s-1) of superheated steam (111 °C) at high pressures (1.42 bar). We find that the thiol coating clearly outperforms the silane coating in terms of both heat transfer and durability. In addition, despite being only a monolayer, it clearly also outperforms the iCVD-fabricated PFDA coating in terms of durability. Remarkably, the thiol layer exhibited dropwise condensation for at least 63 h (>2× times more than the PFDA coating, which survived for 30 h), without any visible deterioration, showcasing its hydrolytic stability. The cost of thiol functionalization per area was also the lowest as compared to all of the other surface hydrophobic treatments used in this study, thus making it the most efficient option for practical applications on copper substrates.

Durable, Ultrathin, and Antifouling Polymer Brush Coating for Efficient Condensation Heat Transfer.

Heat exchangers are made of metals because of their high heat conductivity and mechanical stability. Metal surfaces are inherently hydrophilic, leading to inefficient filmwise condensation. It is still a challenge to coat these metal surfaces with a durable, robust, and thin hydrophobic layer, which is required for efficient dropwise condensation. Here, we report the nonstructured and ultrathin (∼6 nm) polydimethylsiloxane (PDMS) brushes on copper that sustain high-performing dropwise condensation in high supersaturation. Due to the flexible hydrophobic siloxane polymer chains, the coating has low resistance to drop sliding and excellent chemical stability. The PDMS brushes can sustain dropwise condensation for up to ∼8 h during exposure to 111 °C saturated steam flowing at 3 m·s-1, with a 5-7 times higher heat transfer coefficient compared to filmwise condensation. The surface is self-cleaning and can reduce the level of bacterial attachment by 99%. This low-cost, facile, fluorine-free, and scalable method is suitable for a great variety of heat transfer applications.

Microscale investigation on interfacial slippage and detachment of ice from soft materials.

Surface icing is detrimental to applications ranging from transportation to biological systems. Soft elastomeric coatings can engender remarkably low ice adhesion strength, but mechanisms at the microscale and resulting ice extraction outcomes need to be understood. Here we investigate dynamic ice-elastomer interfacial events and show that the ice adhesion strength can actually vary by orders of magnitude due to the shear velocity. We study the detailed deformation fields of the elastomer using confocal traction force microscopy and elucidate the underlying mechanism. The elastomer initially undergoes elastic deformation having a shear velocity dependent threshold, followed by partial relaxation at the onset of slip, where velocity dependent "stick-slip" micropulsations are observed. The results of the work provide important information for the design of soft surfaces with respect to removal of ice, and utility to fields exemplified by adhesion, contact mechanics, and biofouling.

Leidenfrost droplet trampolining.

A liquid droplet dispensed over a sufficiently hot surface does not make contact but instead hovers on a cushion of its own self-generated vapor. Since its discovery in 1756, this so-called Leidenfrost effect has been intensively studied. Here we report a remarkable self-propulsion mechanism of Leidenfrost droplets against gravity, that we term Leidenfrost droplet trampolining. Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and start to gradually bounce from an initial resting altitude to increasing heights, thereby violating the traditionally accepted Leidenfrost equilibrium. We found that the continuously draining vapor cushion initiates and fuels Leidenfrost trampolining by inducing ripples on the droplet bottom surface, which translate into pressure oscillations and induce self-sustained periodic vertical droplet bouncing over a broad range of experimental conditions.